The present invention relates to detecting ultrasonic contrast agents in medical diagnostic ultrasound imaging. Ultrasonic imaging of contrast agents with low transmit powers or low mechanical indices (MI) provides high frame rates by avoiding destruction of contrast agents. However, the returned signals are often weak. With higher acoustic pressures, agents are depleted in the imaging scan plane, and frame rates must be reduced to allow contrast agent to re-enter the scan plane. Higher returned signal levels associated with higher transmitted pressures produce improved image quality, however low frame rates hinder the clinicians ability to maintain a similar scan plane over time and monitor the blood flow in a suspicious area.
Myocardial perfusion, liver pathology, breast pathology, prostate pathology, and portions of the vascular system may benefit from improved contrast agent imaging, especially when high frame rates are achieved using low MIs. For example, the use of contrast agents can improve the assessment of cardiac wall motion and perfusion. Healthy blood flow in the myocardium can take up to 4-8 heartbeats to completely penetrate the entire heart muscle once injected contrast agent arrives in the major heart chambers. The blood filled with contrast agent enters the ventricle, or atria, and the detected agent in this blood is often displayed with extreme brightness due to the high concentration of agent and large blood volume. The blood that later circulates in the myocardium often shows up less bright due to the much smaller blood volume and smaller agent concentration. Small differences in myocardial perfusion, which may indicate areas of ischemia, are difficult to identify since the chambers are typically much brighter and the detected contrast agent in the chambers occupies a large area on the display. The human visual system is often overwhelmed and challenged to easily identify suspicious areas. Different display methods based on simple intensity thresholding may suppress the bright signals in the chambers. However, with this type of method, after the agent has already been detected, signals with similar magnitude may be suppressed in the myocardium as well as the chambers. Critical areas of interest in the myocardium can be suppressed in the display. A technique is needed that suppresses or eliminates the bright signals in larger chambers while maintaining signals in the myocardium.
As another example, differentiation between vessel sizes in abdominal liver imaging is clinically useful. Vascular anomalies, such as a potential hemangioma or heptacellular carcinoma, can be better understood by identifying the structure of vessels feeding a suspicious lesion. The use of contrast agents can improve the imaging of vascular anomalies. However, contrast agent imaging techniques may not specifically detect differences between large and small vessels. Two useful procedures that help a user discriminate between large and small vessels are: (1) to image the initial bolus of contrast agent immediately after an injection and (2) image intermittently while varying the transmit power levels. With the first procedure, the amount of time available is significantly limited. The initial bolus typically transverses the larger arterial vessels within 30-45 seconds. The collection of larger arterial vessels are observed before the contrast agent reaches the smaller micro-vasculature. After the initial bolus travels through the arterial vessels, the blood enters the smaller vessels and then begins to re-circulate. After 30-45 seconds, the image of a liver is often homogeneously bright, making identification of the vessels outside of the tiny microcirculation difficult. With the second procedure, varying the transmitted power changes the amount of bubble destruction in different sized vessels. However, the wash-in of fresh agent into the larger vessels is often short-lived. The period of contrast agent enhancement is diminished due to bubble depletion. When contrast agent is replenished in the microcirculation, the larger vessels again appear similar in intensity, or color, to the smaller vessels. If the transmitted power is raised enough to deplete the contrast agent in certain vessels of interest, the replenishment with new agent occurs quickly, making it difficult to observe the vascular structure. A technique is needed that uniquely identifies different sized vessels throughout the contrast agent examination.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include a method and systems for detecting contrast agents. Differences between different sized vessels throughout the period of contrast agent enhancement are identified without significantly depleting the available agent. Further, the myocardium or microvasculature is uniquely identified from the larger chambers such as the ventricles or atria.
Dual detection paths or processes are used for imaging, such as one for detecting nonlinear contrast agent response and another for detecting differences between the responses to two or more substantially identically transmitted pulses. Where echoes from two or more pulses of acoustic energy are combined to detect the nonlinear response, the nonlinear response may also include signals originating from a loss-of-correlation (LOC) or motion between received pulses. These signals generated from LOC or motion can be produced from agent disruption where a second received echo is different from a first received echo due to a change in a bubble""s shape (i.e., destruction), or from simple spatial translation between acoustic pulses as seen from the same spatial location, respectively. Together the detected differences signals and the nonlinear signals can differentiate contrast agent from tissue. Additional information is gained by detecting signals more responsive to differences due to LOC or motion without more significant nonlinear signal components. Each path detects different relative amounts of nonlinear response and responses caused by detected differences between echo signals of multiple pulses. Various systems and methods for detecting contrast agents where one path preferentially detects differences due to LOC or motion signals and another path preferentially detects nonlinear energy are provided.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.